1. Field of the Invention
The invention relates to ultrasonic measuring method and ultrasonic measuring system for measuring the thickness of a coating material, such as the basis weight of electrode paste applied by coating to a metal foil on an electrode production line in a battery production process, for example, during operation of the line.
2. Description of the Related Art
The battery production process includes a step of producing electrodes from an electrode sheet formed by applying electrode paste by coating to a metal foil on an electrode production line. Since the quality of the electrodes has a large influence on the performance of batteries as final products, it is important, for quality control, to conduct quality check regarding the basis weight (or coating weight) and coating profile of the electrode paste, after it is applied by coating to the metal foil. In some cases, the quality check as described above is desired to be conducted evenly or uniformly over a wide range of the electrode paste of the electrode, on the electrode production line.
Thus, the applicant of this application proposed to make a 100% inspection of the basis weight and coating profile of the electrode paste, on the electrode production line, with respect to all of the electrodes produced on the line, using an ultrasonic measuring system as disclosed in Japanese Patent Application Publication No. 2008-102160 (JP 2008-102160 A), for example. FIG. 24 is an explanatory view showing the ultrasonic measuring system as disclosed in JP 2008-102160 A. As shown in FIG. 24, the ultrasonic measuring system has a pair of ultrasound sending means 81 and ultrasound receiving means 82, which are placed above a measurement object 90, and incident waves sent from the ultrasound sending means 81 are transmitted through the measurement object 90, so that the ultrasound receiving means 82 receives the reflected waves from the measurement object 90.
In the ultrasonic measuring system of JP 2008-102160 A, a propagation time measuring means 83 measures the propagation time of ultrasonic waves propagated through the measurement object 90, based on an incident signal of the ultrasound sending means 81 and a reflection signal received by the ultrasound receiving means 82. Also, temperature measuring means 94a, 94b measure respective temperatures of a liquid phase 91 and a solid phase 92 that constitute the measurement object 90, and a velocity correcting means 85 corrects the propagation velocity of ultrasonic waves propagated, based on the measured temperatures of the liquid phase 91 and solid phase 92, which are measured by the temperature measuring means 94a, 94b. A propagation path length measuring means 86 measures the thickness of the measurement object 90, and a phase-change position of the measurement object 90 as a laminate of the liquid phase 91 and the solid phase 92, based on the propagation time of ultrasonic waves obtained by the propagation time measuring means 83, and a correction value of the propagation velocity obtained by the velocity correcting means 85.
Although calibration of the ultrasound sending means 81 and ultrasonic receiving means 82 is not mentioned in JP 2008-102160 A, calibration of ultrasonic sensors for sending (or receiving) ultrasonic waves is generally conducted in ultrasonic measuring systems of the related art, such as that of JP 2008-102160 A. The calibration is normally conducted while actual measurement of the thickness, distance, or the like, of a measurement object (which will be referred to as “ultrasonic measurement”) is not carried out, for example, before or after the ultrasonic measurement is carried out by an ultrasonic sensor(s), or when the actual measurement is interrupted or stopped, so as to reduce measurement errors in the ultrasonic sensors during measurement.
However, the above-described system of the related art has the following two problems. (1) In the ultrasonic measuring system of JP 2008-102160 A, ultrasonic waves sent from the ultrasound sending means 81 toward the measurement object 90, and ultrasonic waves reflected by the measurement object 90 and received by the ultrasonic receiving means 82 propagate through an air layer as a medium other than the measurement object 90. If the temperature of the air layer is not controlled, the acoustic impedance in the air layer varies with changes in the temperature of the air layer. If the acoustic impedance in the air layer varies, the wavelength of ultrasonic waves propagated through the air layer varies. As a result, the thickness, or the like, of the measurement object 90 cannot be accurately obtained only through correction of the propagation velocity of ultrasonic waves by the velocity correcting means 85.
(2) Also, if the calibration of the ultrasonic sensors is not carried out in real time during actual ultrasonic measurement, the temperature of the atmosphere of the ultrasonic sensors may largely differ between the time when the calibration is carried out, and the time when the actual ultrasonic measurement is carried out. In this case, the intensity of a received signal of a receiving-side ultrasonic sensor that receives ultrasonic waves, for example, may largely change due to the atmosphere temperature.
As one example of the above situation, the graph of FIG. 25 shows test results indicating the relationship between the intensity of the received signal of the receiving-side ultrasonic sensor and the temperature of the atmosphere. In the test, two samples of receiving-side ultrasonic sensors of the same frequency band, which are denoted as Sensor A and Sensor B in FIG. 25, are used. As shown in FIG. 25, the intensities of the received signals of the sensors A, B were both about 825 (mV) when the temperature of the atmosphere was in the neighborhood of 20° C., but the received signal intensities became lower than 780 (mV) when the temperature of the atmosphere exceeded 23° C. It is thus understood that the received signal intensity is reduced by 5% or more, with a temperature rise of 3° C.
As one characteristic of ultrasonic sensors, the ultrasonic sensor is self-heated with a lapse of time in which the sensor is in an operating condition, i.e., the sensor is sending or receiving ultrasonic waves. The graph of FIG. 26 shows, as one example, the relationship between self-heating (the temperature of a receiving-side ultrasonic sensor) and the intensity of ultrasonic waves received by the sensor. As shown in FIG. 26, the temperature of the receiving-side ultrasonic sensor was about 28.5° C. at the time when the sensor starts being operated (t=0 (min.)), but is raised to about 30.7° C. due to heating of the sensor itself, at t=120 (min.) after the start of the operation. On the other hand, it is found that, upon a lapse of two hours from the start of the operation, the received signal intensity is reduced from about 76,200 (mV) down to about 72,300 (mV), namely, reduced by about 5% as compared with that obtained when the operation is started.
In ultrasonic sensors, there is generally a certain correlation between the magnitude of received power of ultrasonic waves (ultrasonic intensity), which is substantially equivalent to the received signal intensity as indicated in FIG. 27, and the wavelength of received ultrasonic waves, as a characteristic of sonic propagation. The graph of FIG. 27 indicates the relationship between the wavelength of the received ultrasonic waves and the ultrasonic intensity. The ultrasonic intensity changes along a normal distribution curve having a peak value at a given wavelength, as shown in FIG. 27. If the wavelength shifts to be a little shorter or longer than the given wavelength corresponding to the peak value, the ultrasonic intensity is reduced largely from the peak value.
Thus, the ultrasonic intensity of the same receiving-side ultrasonic sensor largely changes when the atmosphere temperature of the ultrasonic sensor differs between the time when calibration is performed and the time when the actual ultrasonic measurement is performed, and when the ultrasonic sensor is self-heated; therefore, the wavelength of ultrasonic waves received by the receiving-side ultrasonic sensor largely changes, as is read from FIG. 27. The computation or calculation for the ultrasonic measurement is performed based on the wavelength of the received ultrasonic waves. Therefore, even if calibration is appropriately conducted, the wavelength of ultrasonic waves received by the same receiving-side ultrasonic sensor differs due to a difference in the temperature of the atmosphere of the ultrasonic sensor, and self-heating, unless the calibration is carried out in real time during the actual ultrasonic measurement, and the ultrasonic measurement cannot be accomplished with high accuracy.